As computing devices become more common, the electromagnetic interference (EMI) and radio frequency interference (RFI) caused by one computing device on the operation of another computing device reduces the performance of the other computing device. Such interference may be reduced by special noise reduction casing around the computer devices that deflect any external electromagnetic or radio frequency waves from interfering with the operation and performance of the computer devices. Special paints may also be applied on computing device casings to deflect undesirable electromagnetic or radio frequency signals. Such casings and special paints are expensive and thus not feasible as prices of computing devices reduce.
Noise reduction circuits in computing devices may replace the need for expensive casings and special paints. Traditionally, periodic modulation profiles such as sinusoidal, triangular, and Lexmark profiles have been used to reduce power spectrum peaks generated by processors of a computing device. A lower power peak of the spectrum results in lower EMI interference. Such techniques, however, increase bandwidth of the power spectrum. An increase in bandwidth of the power spectrum may result in interference with neighboring frequency bands causing RFI.
With the inclusion of more and more radios on chip within the free frequency band, multi-radio environment designers face two challenges.
First, the frequency band is becoming more and more clogged with different devices on chip and the surrounding environment. This leads to different devices interacting with each other and causing interference. Such interference can be reduced by shifting the frequency of operation to be away from the interfering channels as shown by FIG. 1. FIG. 1 shows a frequency vs. power plot to illustrate the concept of frequency spreading. To reduce RFI, the operating frequency is shifted as shown by shifting power spectrum 101 to 102. While RFI may be reduced by frequency shifting, the high power peaks of 101 and 102 continue to pose EMI issues.
Second, with increase in frequencies due to abundance of computing devices, reducing EMI to levels specified by regulatory bodies such as the Federal Communications Commission (FCC) is becoming challenging. Traditionally, such challenge is met by spread spectrum techniques that use periodic clock frequency for modulating the power frequency spectrum of a computing device.
The spread spectrum technique increases the bandwidth of the frequency spectrum to decrease the peak power in the spectrum. In FIG. 1, applying spread spectrum technique on the frequency spectrum 105 of a device under test results in a flatter frequency spectrum 104. If a further reduction in peak power is required, a further spread would result in 103. However, reducing the power of 104 to tackle EMI may result in RFI (as shown by 106) due to higher bandwidth of 103. The loss in power to avoid EMI (due to higher power peak) is shown by 105. The higher bandwidth of 103 is caused by enabling a higher spread spectrum technique compared to that applied for 104.